How to hide a mesh on render, but keep hiding polygon it was hiding?
Set the pass index of the shirt to 1 (material tab>settings>pass index). Enable material index for the render layer (render layers tab>passes>material index). Go into the compositor (top bar where it says default, change it to compositing). Select use nodes and set up this node setup:

For the top image input on the Alpha Over node set A: to 0.